The present invention relates to a method and apparatus for continuous electrodepositing of alloys (e.g. Zn-Ni and Zn-Fe alloys) on steel strips.
Steel strips with electroplated coatings of alloys such as Zn-Ni and Zn-Fe alloys are capturing the attention of manufacturers of automobiles, consumer's electrical appliances and construction materials largely because of their good properties such as high corrosion resistance, good compatibility with paints, high press-formability and good weldability. Intensive efforts are being made to commercialize the process of electroplating these alloys, and they have revealed that the principal problem facing commercial alloy plating is how to provide alloy platings of the most uniform composition on steel strips in the largest quantities and at the lowest cost.
The manufacture of steel strips with electroplated alloy coatings generally involves the following problems. (1) In continuous alloy electroplating of steel strips, fluctuation in operating variables cause variations in the composition of the plated alloy and this is often reflected adversely in the quality of the final plating. In particular, if there occurs a change in the distribution of the flow rate of the plating solution at the interface with the work in the cell, variations occur in the composition of the plated alloy, the type of the deposited phase of the alloy, and even in the size or shape of the electrodeposited crystal grains of the alloy and the internal stress in the plated film, and this causes instability in the properties of the plated alloy, which are undesirable for practical purposes.
The distribution of the flow rate of the plating solution varies with the travelling speed of the work. In the actual plating operations, the travelling speed of the work varies unavoidably over a fairly wide range, and as a result, the variations in the distribution of the flow rate of the plating solution are virtually unavoidable.
For these reasons, it has generally been understood that alloy plated steel strips having uniform and consistent performance are inherently difficult to obtain. (2) The recent increases in the capital costs for the construction of electroplating equipment have been so rapid that commercial platers are trying to cope with this problem by minimizing the overall plating length as defined by the number of plating cells times the effective plating length per cell. One approach is to practice plating operations at high current density in each cell.
(i) In the practice of plating operations at high current density, if the distribution of the flow rate of the plating solution at the interface with the work is not uniform, the plated film, whether it is made of a single metal or an alloy, is usually in the form of a dendrite or powdered deposit (commonly called "burnt deposit") and does not have a high degree of smoothness or adhesion to the work. Furthermore, in the practice of high current density plating operations, the flow rate of the plating solution has a certain proper range, and unlike the case of plating of a single metal such as zinc, higher flow rates do not necessarily ensure the best results. More specifically, the distribution of the flow rate of the plating solution determines the final composition of the plated film and the type of the precipitating phase. For example, in the plating of Zn-Ni (5-20 wt % Ni) or Zn-Fe (10-40 wt % Fe) alloys, an excessively small flow rate causes a powdery plate rather than a burnt deposit. If the flow rate is too fast, the plated film has the .eta. phase which impairs its corrosion resistance and weldability.
(ii) If a soluble anode is used in the high current density operation, rapid consumption of the anode necessitates frequent replenishing of the consumed part or even frequent replacement of the entire anode. This causes a prolonged shutdown period and an increase in personnel and cost for replacement operations, which eventually leads to decreased productivity and increased overhead expenses. The use of a soluble anode presents an additional problem peculiar to alloy plating, i.e., difficulty in the control of the composition of the plating bath. For the reasons mentioned above, most of the practical alloy platers operating at high current density are using an insoluble anode.
(iii) However, none of the presently available materials are ideal for use as an insoluble anode. Precious metals (e.g. Pt, Ru, Ir and Au) and their oxides, or lead-base alloys containing at least one element selected from among Ag, Sn, Sb, In, Tl, Hg, As, Sr, Ca and Ba are currently used as insoluble anode materials. Anodes made of precious metals or their oxides are expensive and are used only for plating on electronics materials such as lead frames, and in the plating on steel strips, anodes made of lead alloys is used exclusively. However, this type of anode gradually dissolves in an acidic plating solution as a result of chemical reaction or electrolytic oxidation, and a PbO.sub.2 film formed on the anode surface comes off the anode in particles during the plating operation. The loose PbO.sub.2 particles adhere to the surface of the work and cause "dent marks" as the work is passed between conductor rolls. This is responsible for low yield in the final plating products
(iv) The use of an insoluble anode in plating at high current efficiency causes another problem. Large volumes of oxygen bubbles evolve at the anode and hydrogen bubbles at the cathode (work) surface. Unless these bubbles are rapidly removed from between the electrodes, the plating voltage is increased or the metal film is deposited unevenly or its composition is subject to significant variations.
As shown above, the manufacture of steel strips with electroplated alloy coatings involves various problems and this prevents an expanded use of such strips in spite of the many advantages they have.
While various methods or apparatus have been proposed for use in electroplating operations at high current density, they have their own merits and demerits, as shown below.
(1) Japanese Patent Public Disclosure No. 210984/1982 and Japanese Patent Publication No. 8020/1975 show a plating apparatus of the type depicted in FIG. 1; this apparatus comprises a horizontal plating cell 1 having insoluble anodes 2, 2 formed on the inner surface of both top and bottom walls, and a plating solution is blown into the cell through supply nozzles 3, 3 in a direction opposite to the direction in which the steel strip S travels as indicated by the arrow. This apparatus has some effectiveness in providing a fast and uniform flow rate of the plating solution at the interface with the strip and for preventing the formation of a burnt deposit at high current density. However, gases evolved at the anode 2 and the strip S cannot be sufficiently removed from the small gap therebetween, and PbO.sub.2 particles and other materials that come off the anode surface unavoidably cause the formation of dent marks on the surface of the strip. As a further disadvantage, the anode 2 is an integral part of the inner walls of the rectangular plating cell 1, and this presents appreciable difficulty in repairing the anode which is not "insoluble" in the strict sense and which will gradually wear away in the long run.
(2) Japanese Patent Publication No. 18167/1978 shows a plating method and apparatus of the type shown in FIG. 2; the apparatus includes anodes 2, 2 positioned in a face-to-face relation with the strip S and treating compartments 4, 4 disposed on the back side of the anodes, each anode being provided with a plurality of holes 5 (two holes in the embodiment shown) through which a plating solution is blown onto the strip S in the direction indicated by the arrows. As in the case of FIG. 1, the apparatus shown in FIG. 2 ensures an increased mass transfer to the strip surface and is effective for preventing the formation of burnt deposit and for removing gases evolved between the electrodes. However, the flow of the plating solution being blown normally to the strip S forms an impinging jet stream in the neighborhood of the point where the plating solution strikes the strip. This causes an uneven distribution of mass transfer in the transversal or longitudinal direction of the strip S, and in the case of Zn-based alloy plating, the electrodeposited phase is so affected as to increase the chance of formation of a plated film containing the .eta. phase. As already mentioned, the formation of the .eta. phase is deleterious to the corrosion resistance of the final alloy plated steel strip.
(3) Japanese Patent Publication No. 14759/1982 shows a plating method and apparatus of the type shown in FIGS. 3(a) and 3(b); the apparatus includes an anode 2 that is positioned to face the strip S and which is provided with nozzles 6 in the form of, for example, slit holes which extend widthwise on the anode and through which the plating solution is squirted at high speed against the strip. Technically, this method is based on the same concept as that of the apparatus shown in (2) and cannot be practiced without forming an uneven distribution of the flow rate of the plating solution in the longitudinal direction of the electrodes. If, as shown in FIG. 3(b), a plurality of nozzles 6 through which the plating solution is blown in a direction opposite to the direction in which the strip S travels as indicated by the arrow are arranged in the longitudinal direction of the anode, jets of the plating solution interfere with each other as shown by the arrow heads with dashed lines, and this provides the combination of counter flows and cross flows. The transverse currents flow at an extremely low speed in the horizontal direction in FIG. 3(b), but on the other hand, the flow rate at the point where the plating solution impinges on the strip immediately after it is issued from the nozzle 6 is excessively high. As a result, the composition and the electrodeposited phase of the plated alloy film become uneven not only in the longitudinal direction but also in the transverse direction. Furthermore, the thickness of the electrodeposit is unavoidably non-uniform in oblique directions where the counter flows are combined with the cross flows.
The vertical plating cell shown in FIG. 3(a) has an additional problem; because of gravitational force, it is difficult to keep a jet of the plating solution in contact with the strip S and considerable difficulty is involved in holding the plating solution between the anode 2 and the strip S. This problem is particularly notable on the down-pass side X.sub.1 where a downward drag flow of the plating solution forms due to the descent of the strip. Even if this problem could be avoided, the volume of the plating solution that is necessary to fill the gap between the anode and strip on the downpass side X.sub.1 would greatly differ from that required on the up-pass side X.sub.2, causing a significant difference between the two passes with respect to the distribution of the flow rate of the plating solution at the interface with the strip. Therefore, with the apparatus shown in FIG. 3(a), an alloy plate cannot be deposited in a uniform thickness.
The plating systems shown in FIGS. 1 to 3 are common in that a jet of the plating solution is impinged against the strip surface. In this jet plating system, the plating solution supplied between the anode 2 and the strip S drops into a receiving tank in the form of a large quantity of splash. If the plating solution contains easy-oxidizable ions, for example, Fe.sup.2+ ions (as in the case of Zn-Fe alloy plating), Fe.sup.2+ ions are aerially oxidized to Fe.sup.3+ ions, with the result that the concentration of Fe.sup.3+ ions in the plating solution is increased. The large quantity of splash that continuously drops into the receiving tank has a corrosive action on parts associated with the plating cell, such as the roll drive motor, position detecting instruments, bus bars and carbon brushes on conductor rolls. Furthermore, the splash can endanger the operators working at the plating cell.
Another problem with the jet plating system is that a partial negative pressure develops in the neighborhood of the point where the jet of the plating solution impinges against the strip and increases the chance of ambient air being entrapped in the form of bubbles. If the plating solution contains Fe.sup.2+ ions, this air entrapping accelerates oxidation of Fe.sup.2+ ions to Fe.sup.3+ ions.
A system that could be called "circulation of plating solution in immersion type cell" is shown in literature. This system comprises an immersion type Zn plating cell using an insoluble or soluble anode, and occasionally an ascending flow of plating solution is supplied from the bottom of the cell, thereby providing uniformity in the operating variables of the plating solution such as concentration, temperature and pH. However, this system is intended for the plating of Zn rather than its alloy, and is not based on the concept that a mass transfer should be controlled as uniformly as possible in an area adjacent to the strip surface. The distribution of the flow rate of the plating solution on the strip surface differs not only between the down-pass side and the up-pass side but also between one surface and the opposing surface of the strip. Furthermore, part of the plating solution does not flow in a countercurrent fashion with respect to the travel of the strip. Therefore, this system has not been considered to be capable of providing an alloy electroplate with a uniform thickness and uniform alloy composition in a continuous manner.